The present invention provides a process to prepare various pyridine substituted 5-[4-[2-(alkyl substituted pyridyl)ethoxy]benzyl-2,4-thiazolidinedione derivatives of general formula 1, and their pharmaceutically acceptable salts. These compounds have been found to be advantageous for their therapeutic applications eg. antidiabetic and hypolipidemic, especially as insulin sensitizing agents. Such compounds have been described in patents U.S. Pat. No. 4,687,777 and EP 193256. EP 0508740 discloses Pyridine N-oxide analogues of thiazolidinedione derivatives, including the N-oxide of Pioglitazone (1), having antidiabetic and hypolipidemic activity. U.S. Pat. No. 4,444,779 and EP 008203 discloses new thiazolidinedione compounds, including ciglitazone and their pharmaceutically acceptable salts thereof, which have similar antidiabetic properties.
Diabetes affects a large population and this condition is associated with a number of other complications. Usually, the disease is associated with other disease conditions such as obesity, hyperlipidemia, hypertension and angina. It is a well-recognized fact that improper treatment can aggravate impaired glucose tolerance and insulin resistance, leading to frank diabetes. Thiazolidinediones of formula 1 as well as the novel intermediates 14 & 13 of the present invention are useful in the treatment of diabetes, and affect lipid metabolism.
Pioglitazone belongs to the thiazolidinedione group of antidiabetics. Later it has been found that its antidiabetic effect consists in reducing insulin resistance, thereby improving glucose homeostases without increasing insulin secretion, unlike most other antidiabetics. For these extraordinary characteristics this product is of great importance for the treatment of non-insulin dependent diabetes mellitus. Combination with insulin or other antidiabetics can further increase its effect.
Methods for production of various thiazolidinedione derivatives are described in U.S. Pat. No. 4,687,777; Drugs of Future, 15, 1080 (1990); Chemical and Pharmaceutical Bulletin, 30, 3563 (1982); 30, 3580 (1982) and 32, 2267 (1984). These methods invariably comprise low temperature diazotisation, condensation with lachrymetric and readily polymerizable reagent acrylic ester in the presence of a copper catalyst by Meerwein arylation reaction to give a haloester, reacting it with thiourea to give an iminothiazolidine and finally hydrolyzing the same to get the required thiazolidinedione derivative. These methods include multistep synthetic processes and sometimes it is difficult to control Meerwein reaction at industrial scale, since it is an exothermic run-away type of reaction accompanied by the generation of a large amount of nitrogen gas, which is difficult to handle. Moreover, due to byproduct formation, purification becomes cumbersome. Besides, special measures are required in the Meerwein reaction for elimination of an extremely bad odour of acrylic acid ester, which must be used in excess. The disposal of excess material, along with heavy metals, requires additional effluent treatment protocols. These issues make the known route disadvantageous both technically and commercially.
Subsequently, new synthetic strategies have been reported in EP 0257781, which might lead to side product(s) eg. 2-vinyl-5-ethyl pyridine from tosylates, and require high pressure Raney Ni conversion of cyanide to formyl group. The purification of the intermediates is also difficult in this process. In an yet another invention, microbial reductase has been employed to obtain pharmaceutically active thiazolidine derivatives (WO 9310254).
Recently, Pioglitazone oxygenated metabolites have been patented (WO 9322445) as potentially useful compounds for the treatment of diabetes and as insulin sensitizing agents (J. Med Chem., 1996, 39, 5053). PCT Patent No. WO 93/13095 describes the use of cobalt ion, a ligand and a reducing agent to convert the final step reduction of 5-methylene thiazolidinedione to saturated analogues.
U.S. Pat. No. 5,594,015 describes the new use of Pioglitazone for the treatment of Psoriasis. Various other strategies to synthesize Pioglitazone are disclosed in Patents EP 0506273. As discussed above in the prior art, the known method to prepare compounds of general formula 1, in particular, Pioglitazone, involves technically difficult procedures to handle bad odour, low temperature diazotization, evolution of large excess of gas and special precautions to handle effluents.
Besides, above mentioned procedures lead to the formation of unwanted impurities, the removal of which is a time consuming process. Environmentally also, it requires evolution of HBr gas, which requires upstream processing and consequent additional cost.
Subsequent to our provisional application a recent published article in Organic Process Research Development, 2002, 6, 721-728 describes a method for preparing compound of formula 14, when X═O